1. Technical Field of the Invention
The present invention relates generally to the wireless telecommunications field and, in particular, to a method and apparatus for synchronizing a plurality of private radio systems that are commonly using a mobile communications air interface and air protocol.
2. Description of Related Art
The commercial utilization of digital mobile communications systems for public telephony has grown tremendously during the past decade. For example, since the mid-1980's, a large number of digital cellular communications systems have been fielded for public use throughout the world, such as the Global System for Mobile communications (GSM) in Europe, the Personal Digital Cellular (PDC) System in Japan, and the Digital Advanced Mobile Phone System (DAMPS) in North America. However, although many of the technical specifications for these systems are different, the GSM air interface and protocol, which were created in Europe, are being used in many of the other cellular systems deployed throughout the world.
In business and residential environments, users of telephony services have limited mobility and, therefore, can be offered services not normally available to the highly mobile users of cellular networks. For example, U.S. Pat. Nos. 5,428,668 and 5,535,259 disclose a private radio base station, which is not an integral part of a disclosed cellular network, but is connected directly to a public switched telephone network (PSTN). When a mobile cellular terminal comes within radio range of a "home base station" (HBS) of the private system, the mobile terminal is converted operationally to a cordless phone. Using this cordless mode of operation, a user can realize numerous advantages over the cellular mode, such as the lower PSTN rates incurred, higher quality voice communications, and a substantially longer standby time.
Many similar and other advantages would be realized if a private radio system were to have an air interface that is compatible with a cellular network air interface. For example, a mobile terminal could be converted into a dual-mode cellular-HBS portable terminal, by downloading software from the mobile network or private system to implement the cordless mode of operation. Additionally, the private radio system's base station existing hardware could be used in such a dual-mode cellular-HBS operation, provided the base station's transmit and receive frequencies are exchanged.
However, an interference problem arises if a private radio system's HBS uses the same frequencies as those used by a cellular network, and their coverage areas overlap. Typically, in this situation, an operator would be required to dedicate one or more frequencies exclusively for use by the HBS. Consequently, even if the HBS coverage area were to overlap with that of the cellular network, the dedicated HBS frequencies would not interfere with those used by the network. Therefore, the HBS frequencies should be orthogonal to the cellular network's frequencies, and adjacent channel interference can then be avoided.
Frequencies in the radio spectrum are scarce, and there are not many carrier frequencies available for dedicated HBS use. Consequently, another interference problem arises if the coverage areas of different HBSs overlap. Since the different HBSs typically do not communicate with each other, it is important to appropriately allocate the HBS channels. Adaptive channel allocation techniques may be used to divide the available channels among the different HBSs, so that mutual interference will be minimized. However, an adaptive channel allocation technique is considered to be optimal only when the allocated channels are orthogonal to each other. For example, a GSM channel is defined by both a carrier frequency and a time slot. Frequency orthogonality means that a strict frequency spacing is maintained between consecutive carriers, so there is a minimal amount of adjacent channel interference. In the GSM, the specified accuracy for a base station carrier frequency is better than 0.05 ppm. Time slot orthogonality means that consecutive time slots do not overlap. The transmission of the TDMA bursts in a GSM cell is synchronized to the timing of the base station, which acts as a master reference. In this way, interference caused by overlapping bursts transmitted by different mobile terminals is prevented.
In that regard, another synchronization-related problem arises when HBS-based private radio systems are used. The cost of obtaining the highly accurate frequencies and timing needed to minimize interference is far too expensive for typical private radio systems. For example, in order to obtain the highly accurate carrier frequencies required in cellular networks, relatively expensive oven-controlled signal reference sources are used. For the typical private radio system, the use of such a high quality reference source would be too costly. Similarly, in order to obtain appropriate time synchronization, cellular base stations adjust all of the uplink and downlink channel timing. In order to suppress interference from other (non-synchronized) base stations, cellular networks employ frequency cell planning techniques to ensure that co-channel interference sources are separated by an appropriate distance.
On the other hand, in a private radio system environment, the HBS units are not coordinated with each other. Consequently, it is not technically (or economically) feasible for a private radio system user to institute frequency cell planning or relative time adjustments for the different HBSs in that or any nearby system. It is possible to use adaptive frequency planning techniques in private radio systems. However, the number of frequencies available for HBS use are usually insufficient to achieve desired frequency reuse distances in those areas where many HBSs are located close together. In that event, many of the HBSs would have to share the same carrier frequency. Therefore, in order for private radio system HBSs to obtain any desired channel reuse, time slot synchronization techniques could be employed so that a number of users could use the same carrier frequencies without experiencing traffic "collisions". Because of over-riding technical and cost considerations, it is preferable to time synchronize a plurality of private HBSs without having to provide interconnecting communications links between them.
A major frequency synchronization problem arises for users of private radio systems, because in order to minimize interference, the private HBSs should use carrier frequencies that are orthogonal not only to the surrounding cellular network frequencies, but also the frequencies of nearby HBSs. However, it would be far too expensive for private system users to incorporate the highly accurate reference sources typically used in cellular networks (e.g., GSM) into the private radio base stations or such consumer products as the private radiotelephones.